Coda fanfiction is a type of fan - created story. It often takes the events of an existing work, like a movie, TV show, or book, and continues or expands on them in a new and creative way. For example, if a TV show has a big finale, a coda fanfiction might explore what happens to the characters a few years later, filling in the gaps that the original left open.
Well, a coda in fanfic is sort of like a bonus part. It can show what happens after the main events of the story, maybe giving us a glimpse into the future of the characters or tying up some loose ends. Sometimes it's just for fun or to satisfy the readers' curiosity.
The JJBA manga is indeed finished. It had a great journey and wrapped up with a satisfying conclusion for fans. However, there's always the possibility of spin-offs or related works in the future.